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Author Topic: Isolating the Prop Tube from the Hull...  (Read 616 times)

DJW

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Isolating the Prop Tube from the Hull...
« on: March 24, 2021, 10:20:30 pm »

Hi All

I was wondering whether anyone has tried to isolate the prop tube from the hull..?


Aim would be to reduce vibration / noise transmitting to the hull. I was talking to UKMike earlier about the bobbin mounts he uses to mount outrunner motors, I think that's a really good approach to mechanically isolate the motors. I'm thinking that the prop tube could be embedded in a Sikaflex or similar bedding compound as it exits the hull rather than rigid epoxy which gives a great seal, but also transmits any vibration or mechanical noise from bearings directly to the hull which then amplifies them...

I'm interested particularly in a solution for an exposed shaft with P brackets. So the P bracket would need the same treatment. A side effect would be that the P bracket could be inserted from inside as the cutout wod be larger, this would allow a more robust top end of the P bracket to be fabricated. This I think would be helpful.

The result would be the whole drive train mechanically separated from the hull. I guess a solid coupling could also be used between motor and shaft.

I think this could reduce the mechanical sounds of a pair of electric motors running at speed.

Any comments...?
Best regards to all.
David.

Colin Bishop

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Re: Isolating the Prop Tube from the Hull...
« Reply #1 on: March 24, 2021, 10:42:49 pm »

If you have your driveline properly set up then there should not be any vibration issues.

Colin
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derekwarner

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Re: Isolating the Prop Tube from the Hull...
« Reply #2 on: March 24, 2021, 11:09:31 pm »

Hullo David.......always a few things to consider with such an issue


1. Minimise any form of out of balance component elements from rotating driveline



2. Harnessing a vibration of the drive train will transfer any out of balance resonance to the mass of the structure [hull] which could amplify the sound, however minimise the amplitude of the cause


3. Adding any form of resilience to the drive train could minimise the amplitude of the sound, but increase the actual vibration


4. This is why automobile manufacturers attempt to minimise vibration, then, if necessary add sound [resonance] adsorbing panels in their builds  O0


I still think 1. + 2. are still preferable to 3. for our hobby builds, ..........1. is the task [and solution] of the manufacturers of rotating machinery


[Just imagine a Supervisor in Rolls Royce or GE aircraft turbine engines quality Department saying......"if the noise of the vibration is too high, just add a few dollops of Silastic to the holding down bolts"  %)  ]

Derek
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npomeroy

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Re: Isolating the Prop Tube from the Hull...
« Reply #3 on: March 25, 2021, 06:18:40 am »

I would have thought there was a case for vibration damping for noise reduction.  All motors and certainly propellers make some noise.  It depends on the geometry w.r.t. resonance whether any flexible sheath absorbs or amplifies oscillations.  In my experience with scale helicopters, while the main driving components are rigidly supported, we often use silicone buffers between the chassis and the fuselage shell.  And the servos are generally rubber boot mounted.
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DJW

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Re: Isolating the Prop Tube from the Hull...
« Reply #4 on: March 25, 2021, 07:58:44 am »

Hi All


Thanks very much for the input.  Always a good idea to get something like this 'sanity checked'.


Maybe 'vibration' was the wrong word for me to use. This is absolutely not an alternative to good alignment of rotating parts, and I agree that if there is misalignment this could make the problem (a lot) worse. I did mention a solid coupling, this would help to keep the rotating mass aligned.


Some additional information.  I'm currently building an Aquarama, and am contemplating a second, larger scale.  So that's two powerful brushless outrunners, the a ball race at the top of each shaft plus a thrust bearing.  Its the bearings that even when perfectly aligned do make a noise, maybe not a vibration.  Then two of these systems would spun at 10k plus and attached to a large hollow wooden case...  Sounds like a speaker system to me.  So I think its how to set and maintain shaft alignment while stopping the hull amplifying the bearing whine.  I'd have thought some experimenting with the right compound and amount of it would be important.


At the end of the day, its trying to stop the model sounding like a model...


Does that make more sense..?


Best regards to all.
David.

ukmike

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Re: Isolating the Prop Tube from the Hull...
« Reply #5 on: March 25, 2021, 11:30:45 am »

Morning David.


I know that is no chance that you would have misaligned the transmission system.


You sound very much like myself in that you hate unnecessary noise and the more isolation of the transmission you can get the better.


On my big Riva I have done as much as possible to alleviate unnecessary noise, or so thought.


Your idea re the Sikaflex is both simple as sound, Sikaflex is very good stuff so go ahead and do it.


All you will get are positive results, I'm sure of that.


Mike.
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Tim_M

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Re: Isolating the Prop Tube from the Hull...
« Reply #6 on: March 25, 2021, 04:31:22 pm »

This is an interesting topic and it's nice to see some 'science' being applied. I worked for years as a vibration and dynamics engineer. Here's a few random thoughts that might (or might not  {-) ) help you.
You've got two things to do. First is to reduce the source of any vibration and, as you say, getting the driveline straight is the key. Don't use a UJ because that will cause vibration. Now, when it comes down to isolating the vibration, that's a bit more tricky. Your 'once per rev' out of balance vibration will have a frequency of 167 Hz (cycles per second in old money) at 10 000 rpm. That's quite a low pitched noise. Any decent motor will be balanced and the shaft itself is so small that out of balance won't be much of an issue. The props should normally be balanced before use. Another source of vibration is the 'cogging' torque - the kick that the motor gets several times in each revolution. That will be a higher pitch noise (depending on the number of poles in the motor). Finally, the bearings, if you use ball bearings, will produce some noise at even higher frequency - the sound you don't like.


The trick to isolating vibration is to introduce a soft element between the drive mechanism and a very stiff base. That last bit is important. If you soft mount a motor on a floppy hull structure the isolators won't work. The problem you will face, especially at model scale, is that most of the 'soft' materials you are thinking about (like silkaflex) will actually be too stiff when installed to do any good. If you mount the motor of a soft mount the arrangement will have a natural frequency (often called resonant frequency). The value depends on the mass of the motor and the stiffness of the mount. Look out, maths coming up. The natural frequency is worked out using f = 1/2pi sqrt(k/m). Oh blimey. What that shows is that as the mass gets bigger, the natural frequency goes down. As the stiffness get higher, so does the natural frequency.


Now you have your motor sitting on its mounts. Vibrations at the natural frequency will be amplified! If the mount is not damped (for example, it's a metal spring) then the thing will fly all over the place and tear itself to shreds.  Vibrations above the natural frequency will be isolated - the higher the frequency, the more the isolation. Vibrations below the natural frequency will not get isolated at all. [/size]Disappointing, isn't it?

[/size]So, what would I do in practice?
[/size]First, I think soft mounting the P bracket would be a very bad idea. It would allow the prop to move (vibrate) too much. I looked at some pictures of Riva models and there's a number of different, poor, mounting arrangements. On the real boat, the P bracket is up behind the prop and this is also the best place to put your thrust bearing. There's an advantage to having the thrust bearing there as it will tend to lift the bow rather than forcing it down by reacting the thrust near the motor. I would make the hull as stiff as you possibly can where the P bracket attaches (over a large area) and hard mount the bracket. You could soft mount the motor but you will need proper isolation mounts (cheap but tricky to work out which ones you need). Just adding grommets to screws will achieve little or nothing. I would use soft motor mounts anyway to allow some end float in the motor so that you allow the thrust bearing in the P bracket to work and don't add any end loads to the motor (that will make the bearings run quieter too). Again, make the area where the motor sits as stiff as you can. Stiff structures can't transmit vibration, because they are too stiff to move. Using a soft seal around the prop tube won't hurt but I doubt you will see much benefit. The shaft shouldn't be touch it anyway. I'm not even convinced you need a bearing at the end of the shaft tube. If it's aligned properly with the motor, let the motor bearing do the job and then you won't have the two bearings fighting each other (but you'll need something to keep the water out).

[/size]If you get the driveline truly straight and install it as an assembly, letting everything find its own place, most of your issues will go away. If you are worried about bearing noise, don't use ball bearings. Use plain bearings, as used in precision, quiet, machinery.

[/size]The good news is that the hull, being double planked, will be stiff and well damped. It shouldn't act very well as a sound box. I think if I were really looking to reduce the noise of the drive line, I would just make a good size soundboard and try some different arrangements of mount to see just what works with your hardware. I'd also be tempted to add a good sound module to the boat and treat any residual whine as representing the turbos winding up :} .


It will be interesting to see what you eventually come up with  :-))
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DJW

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Re: Isolating the Prop Tube from the Hull...
« Reply #7 on: March 25, 2021, 05:20:42 pm »

Hi Tim


Wow..!  Some interesting and very helpful comments there, thanks very much.


I'm going to go over what you've written a few times to get the implications straight in my head, then respond.


I do like your lateral thinking too...  'If you don't like the sound of ball bearings, don't use them...'   :-))


Thanks again.
Best regards
David.

DJW

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Re: Isolating the Prop Tube from the Hull...
« Reply #8 on: March 25, 2021, 07:53:03 pm »

OK...  So given the solution I'm looking for is based around exposed shafts, using P brackets, a couple of things...


It sounds like UKMikes approach of using the rubber isolation (bobbin) mounts is validated, and even better as they are 'tunable' in that they are recessed at the mount end. The amount of bobbin exposed would vary the natural frequency absorbed I'd have thought.  So rather than attempt to calculate, they can be tuned in situ.


Then a solid coupling sounds the ticket, ensuring perfect alignment.


Then the fun starts.  My original post suggested a conventional prop tube, but isolated from the hull.  I think what Tim is saying is that ideally, there would be no bearings in the tube, just seals...  So that isolates the rotating shaft from the tube, (rather than the tube from the hull) then just epoxy in the tube to the hull as normal.  And so no bearing noise, and no conflict between motor and shaft bearings. The seals would have enough tolerance to absorb slight movement of the motor on its mounts. So the prop tube becomes more of a stuffing box, just a water seal...


Then the solid mounted P bracket which has a bearing, and takes the thrust.


Two brave moves required there (for me) a bearing free prop tube...  And putting 2 to 3Kw onto the P brackets...


Have I got that vaguely right Tim..?
Best regards
David.

Tim_M

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Re: Isolating the Prop Tube from the Hull...
« Reply #9 on: March 26, 2021, 12:02:37 pm »

You have indeed understood well! People say I get too excited when I explain things but, generally, I get understood!
Do bear in mind that the things I described are a watered-down version of the 'pure' engineering. It gets complicated very quickly in real life so you'll need some experimentation. That's the fun part of modelling, in my book. As long as you grasp the idea of natural (resonant) frequency then you know what to look out for.


You can get the isolating bobbins in various sizes, styles and hardness. I would shop around for the best deals as any sold for 'modellers' will carry an outrageous mark-up. Do bear in mind you don't want to motor wobbling around too much as the other end of the shaft is going into a fixed bearing.


I wouldn't worry about putting the thrust through the P bracket as long as it's a proper job. Look at the struts used on high powered racing models. Same idea, different name.


Wish you luck with all of this. The Aquamarina is a beautiful model, well beyond my skill level!
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Stuw

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Re: Isolating the Prop Tube from the Hull...
« Reply #10 on: March 31, 2021, 09:26:58 pm »

Interesting stuff. Iím of the view with my brushless inrunner setup to not have any rubber mounts, just all aligned as much as possible. If the boat resonates then maybe try to reduce this with adding wood to the inside of the hull to reduce skin resonance etc or just live with it.


This is all hypothetical on my part as Iím still putting the pieces together.


With brushless over glowfuel I figure rubber mounts arenít as critical but I may be very wrong!  %%
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DJW

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Re: Isolating the Prop Tube from the Hull...
« Reply #11 on: April 01, 2021, 02:13:40 pm »

Hi Stuw


I don't think you're wrong at all. On my current build I've braced areas and I'll be adding some sound absorbing neoprene to help kill motor sounds.  I started the thread here as its about 'R&D', so just exploring options really. How to stop the sounds in the first place rather than damp them once produced.


Some very interesting points raised I think, and I will be trying out some tests before committing into a build.  A couple of observations having pondered on the issues raised:


1. While testing existing build on my desk, I had motors mounted on quite heavy aluminium angle plate, ESCs and Lipos, running no load at all.  Significant noise from the motors, while they run very smoothly, the bearing noises are amplified when in contact with the desk.  So this is not any alignment / vibration issue, pure motor sounds. I put a piece of neoprene under the mount, sound goes away. For me that confirms a rubber mount approach for the motors will yield results.


2. A conventional setup needs a bearing of some sort top and bottom of the prop tube, and usually a seal.  But the P bracket approach with an exposed shaft, the P bracket could be considered to be the lower (shaft) bearing.  And as Tim pointed out, there is a risk of the top bearing 'fighting' with the motors own bearings if alignment is not 100% perfect.  So (assuming a solid coupling) why not consider the motors own bearings as the top 'tube' bearing, this can positively help with maintaining alignment.  Then its just a case of using seals in the prop tube to keep the lake on the outside....


I think its well worth trying and will be doing so.  And it may be useful during construction to just put plain bearings in to get the alignment right in the first place, then remove them and replace with seals once tube is bonded in.  Then if it doesn't work, the plain bearings can always go back in... If it does work, the P bracket may be the only surface to surface contact of the whole drive train, and it could use a delrin or similar bearing / bush, water lubricated.


Or it may be garbage... We shall see....    %)
Best regards
David.

Stuw

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Re: Isolating the Prop Tube from the Hull...
« Reply #12 on: April 01, 2021, 04:42:23 pm »

Itís an interesting area to look at. I see your point about noise transmission. My brain worries about movement caused by the flexible mount leading to wear/vibration etc further down the line. Hence your flexible proptube mount idea.


Food for thought definitely. Will see how mine goes. Iím making a removable motor mount so could change the setup. I think the rubber mounts that Iíve seen on a build log look good. My mounting is under the motor though versus the front face of the motor being attached to the flexible mount and then to a bulkhead in the same axis.


Your builds and those with the mounts are seeking noise reduction having made such fine models to eliminate non realistic noises. My old Lesro Javelin is my first project and Iím just trying to get her running. As it was designed for noisy glowfuel motors a bit of resonance is required perhaps  {-)


Keep the info coming if you do some more work on this.
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